From 1979-1984 the flywheels in the 302 Mustangs used a 10" clutch. The 1985 and newer used a 10'5" clutch. The flywheel diameter was the same throughout. From '79-'81 the flywheels were of the 28oz version.

If you want to use an 80's flywheel there is a 28oz unit already available('79-'81). If you want the 10'5" clutch(recommended for performance apps) then do what the earlier poster recommended. If you just want to bolt it on and go without drilling out material and weakening the flywheel's integrity get a '79-'81 unit. I have used a T5 and T5 bellhousing with both of these on a 5.0 and the bolt pattern is the same for a 351. Your only issue would be what type of clutch mechanism you want to use.

I'm not sure about the configurations of the '78 and older units but they may work too.

All that said I don't recommend a T5 behind a 351. Even the best is only rated at 330 ft/lbs of torque and the stock one is rated at 265. The only stock ones that aren't are the '91 and newer units, 305 and the Cobra units, 310.

The top-loader is a super strong unit but the gearing choices are only two; Wide ratio and close ratio. Wide ratio has a first gear around 2.95 and the close ration is way lower. Neither one is well suited for a tall rear gear. The stock t-5 has a first gear of 3.35 but is of the lower rated torque of 265(unless it's a newer model or the heavy duty trans). The heavy duty T5 is rated at 330 but it's first gear is 2.95 so you're back where you were with the Top Loader and the Top Loader is way stronger. having a fifth gear is great for the highway though.

Whatever you do, good luck with it.